Dynamometric key

ABSTRACT

The invention concerns a dynamometric key designed in particular for fixing an insert ( 22 ) on a handpiece ( 23 ). Said dynamometric key is characterized in that it consists of a grip element ( 3 ), and a securing element ( 5 ), said two elements being linked in frictional rotation via a spring ( 1 ) whereof the end coils have been assembled, so as to provide it with a globally toroidal shape.

[0001] The present invention relates to a dynamometric key and moreparticularly to a key of this type intended to ensure tightening andloosening of an insert on a dental handpiece.

[0002] Dynamometric keys used in industry, which are adapted to deliverconsiderable tightening and even loosening moments, are known. Suchdynamometric keys are constituted by complex mechanical elements whichare easy to employ in devices of respectable volume but which areparticularly difficult and expensive to manufacture as soon as it isdesired to transpose them to a smaller scale.

[0003] Patent U.S. Pat. No. 6,021,694 in particular discloses adynamometric key intended for surgical use which is essentiallyconstituted by a tubular element containing a linear helicoidal springof which the level of compression is determined by means of an adjustingscrew. This compression spring acts by friction on the outer part of theball bearing housing, which retransmits to the end of the key a momentdetermined by its level of friction. Such a device is difficult to usein certain domains of applications and in particular in that of thedental domain by reason of the considerable lever arm constituted bythis device which gives it a large volume which is incompatible with thetechniques used.

[0004] The present invention has for its object to propose adynamometric key which is particularly compact and simple inconstruction and which, moreover, makes it possible, during use, toprotect the hand of the practitioner who is actuating said key, from theend of the tool.

[0005] The present invention thus has for it object a dynamometric keyintended for tightening/loosening an element to be fixed, particularlyan insert on a dental handpiece, characterized in that it is constitutedby a grip element and a securing element which is linked at least inrotation with the element to be fixed, the grip element and the securingelement being linked in frictional rotation via a spring of helicoidaltype of which the end turns have been assembled so as to provide it witha substantially toroidal shape, the two parts of the same turn, locatedon either side of a median plane, are inclined on the same side withrespect to the radial axis of the spring.

[0006] The turns of the spring, when the latter is in the free state,will preferably have a flattened shape along its radial axis, thisflattened shape being able to take the form of an ellipsis inparticular.

[0007] The securing element may preferably be constituted by acylindrical element comprising a cylindrical outer groove in which thespring is fitted, the base diameter of the groove being slightly greaterthan the inner diameter of the spring when it is in the free state.

[0008] The grip element may have a cylindrical recess hollowed outtherein, of which the diameter defines, with the diameter of thesecuring element, a spacing less than the height of the turns when thespring is in the free state, so as to compress them.

[0009] In a form of embodiment of the invention, the grip element willcomprise two parts, namely a substantially cylindrical upper part and alower part substantially in the form of a bell.

[0010] The internal volume of the bell-shaped part may preferablyenvelop the tool, whatever the positioning thereof inside thedynamometric key.

[0011] In this form of embodiment, the immobilization, in the axialsense, of the securing element with respect to the grip element, will beensured in particularly simple manner by the spring itself, which, apartfrom the very simplicity of this mechanism, allows rapid assembly anddismantling of the device.

[0012] According to the invention, it has been observed that the momentwhich it is necessary to exert in order to rotate the securing elementby driving it in rotation by means of the grip element, was greater inone direction of rotation than in the other. Under these conditions, itwill be understood that, if the direction of rotation generating thelower moment corresponds to the moment for tightening the insert on thehandpiece, the other direction of rotation will apply a greater momentwhich, if it is well adjusted, will ensure loosening of the insert fromthe handpiece.

[0013] It has been observed that, for a spring of given dimensions andmechanical characteristics, the tightening/loosening moment wasincreased by giving the housings, between which the spring is disposed,a dimension ensuring a compression of the turns thereof in asubstantially radial direction.

[0014] A form of embodiment of the present invention will be describedhereinafter by way of non-limiting example, with reference to theaccompanying drawings, in which:

[0015]FIG. 1a is a plane view of a helicoidal spring according to theprior state of the art, of which the two ends have been secured so as toprovide it with a globally toric shape.

[0016]FIG. 1b is a plan view of a spring used in the dynamometric keyaccording to the invention.

[0017]FIG. 1c is a side view of the spring shown in FIG. 1b.

[0018]FIG. 2 is a view in axial and longitudinal section of thedynamometric key according to the invention.

[0019]FIG. 3 is an enlarged partial view of the upper part of thedynamometric key shown in FIG. 2.

[0020]FIG. 4 is a view in cross section of the dynamometric key shown inFIG. 2 along line IV-IV thereof.

[0021]FIG. 5 is a curve representing the variation of the difference inmoment transmitted by a spring in its two respective directions ofrotation as a function of the deformation exerted on its turns in theradial sense.

[0022]FIG. 6 is a view in partial longitudinal section of a handpiece,on the insert of which the dynamometric key according to the inventionis positioned.

[0023]FIG. 1a shows a helicoidal spring 1′ of conventional type of whichthe ends have been assembled so as to provide it with a globally toricshape. It is seen that the two half-turns 1′a and 1′b which are locatedon either side of a median plane P (FIG. 1c) lie substantiallysymmetrically with respect to a radial axis Or when the spring isobserved in plan (FIG. 1a), and the respective half-turns 1′a and 1′bform equal angles α′1 and α′2 with respect to axis Or.

[0024] As for the device according to the invention, it resorts to aspring of the same type likewise of globally toric form but in which thetwo half-turns 1 a and 1 b are inclined on the same side of the radialaxis Or. According to the invention, as shown in FIGS. 1b and 1 c, therespective half-turns 1 a and 1 b form different angles α1 and α2 withthe radial axis Or.

[0025] It has been ascertained that the springs of this type, when theywere used as intermediate elements to transmit a torsional moment, hadthe property of transmitting, before slide, a moment C of a certaingiven fixed value when their rotation was effected in one direction anda moment of another, lower, given fixed value C′, when the rotation waseffected in the other direction, the two absolute values C and C′ ofthese two moments being, of course, a function of the materials withwhich they are placed in contact. It has been ascertained that thehigher moment C was transmitted when a relative movement of the spring 1was made with respect to the component with which it is in contact, inthe direction of arrow C, i.e. when the relative displacement of thespring was effected in the direction followed when passing from an innerpart of a turn and going towards its outer part, i.e. in the directionof arrow C in FIG. 1b.

[0026] Springs of the same nature are known from the prior state of theart and are used for numerous applications, but they have not been usedup to the present time for transmitting determined respective fixedmoments of rotation.

[0027] The dynamometric key according to the invention, which is shownin FIGS. 2 to 4, is essentially constituted by an outer ring 3 made ofaluminium, which constitutes a grip element, and by an inner core 5 madeof stainless steel, between which is arranged a spring 1 of the typedescribed previously, and which is shown in FIGS. 1b and 1 c.

[0028] The ring 3 and the core 5 might, of course, be made of any othermaterial and in particular of plastics material or ceramics.

[0029] The inner core 5 is constituted by a cylindrical element whichhas a circular groove 7 hollowed out therein whose inner diameter d issuch as to receive the spring 1 while exerting a slight stress thereon.To that end, the diameter d will be slightly greater than the innerdiameter d′ of the spring 1 when the latter is in the free state. Abovethe groove 7, the core 5 comprises a first boss 9 followed by a secondboss 11 of larger diameter and of lesser thickness. Below the groove 7it terminates in a circular plate 13. In its upper part, the core 5comprises a slot 20 which is intended to ensure the mechanical link inrotation of the dynamometric key according to the invention with thedevice for tightening/loosening an insert 22 of a handpiece 23 forexample, as shown in FIG. 6.

[0030] The outer ring 3 comprises a first part 3 a (upper in thedrawing) of substantially cylindrical shape, followed by a second part 3b (lower in the drawing) in the form of a bell. The cylindrical part 3 areceives the core 5 which will be previously provided with the spring 1.To that end, it comprises cylindrical upper recesses 15 and 17respectively intended to receive the bosses 9 and 11. Under the recess17, a cylindrical housing 18 has been made, whose diameter D cooperateswith the diameter d of the core 5 in order to define therebetween aspaced apart relationship δD=(D−d)/2 whose value determines the state ofcompression of the turns of the spring 1.

[0031] In effect, it has been ascertained that, when the turns ofsprings of the spring 1 type were subjected to a radial compression, notonly the absolute value C and C′ of the moments of rotation transmittedwas increased, but the difference δC=C−C′ of these moments for oppositedirections of rotation was also varied. By measurements, it wasestablished that the value of this difference δC was connected with thevalue of the compression of the turns of the spring δD=(D−d)/2, and thisfor a spring 1 of determined mechanical characteristics, depending on afunction of which the course is shown in the curve of FIG. 5.

[0032] It has also been ascertained that, for a given radial compressionlevel, the transmitted moments C and C′ were higher when the diameter ofthe wire constituting the turns was greater.

[0033] By adjusting the spaced apart relationship δD=(D−d)/2, and thisby adjusting the dimensions of the diameters D and d, it is thuspossible to control the difference δC which exists between the values ofthe moments which may be applied with the dynamometric key according tothe invention.

[0034] The latter is particularly interesting to ensure a tightening,with a determined given moment C′, of an “insert” 22, i.e. of a tool ona dental handpiece 23.

[0035] As shown in FIG. 6, it suffices to that end for the practitionerto introduce the insert 22 in the slot 20 so as to ensure its link inrotation with the core 5, then to animate the ring 3 by a movement ofrotation. To that end, steps will be taken for the spring 1 to bepositioned on the core 5 so that, by rotating in the direction oftightening, the moment C′ of lower value is applied to the insert, andthat, by rotating in the direction of loosening, the moment C of highervalue is then applied, so as to ensure unscrewing.

[0036] It will be noted that the dynamometric key according to theinvention presents the advantage, by a simple turn of the spring 1, ofallowing its direction of tightening to be reversed, which makes itpossible to adapt it easily and rapidly to an insert/handpiece device ofwhich the direction of the pitch is reversed with respect to the usualdirection.

[0037] Furthermore, the shape of the dynamometric key according to theinvention allows the practitioner to operate in complete safety, insofaras his hand controlling the rotation of the grip element is protectedfrom the insert by the bell-shaped part 3 b.

1. Dynamometric key intended for tightening/loosening an element (22) tobe fixed, particularly an insert on a dental handpiece, characterized inthat: it is constituted by a grip element (3) and a securing element (5)which is linked at least in rotation with the element (22) to be fixed,the grip element (3) and the securing element (5) being linked infrictional rotation via a spring (1) of helicoidal type of which the endturns have been assembled so as to provide it with a substantiallytoroidal shape, the two parts (1 a, 1 b) of the same turn, located oneither side of a median plane (P), are inclined on the same side withrespect to the radial axis (Or) of the spring (1).
 2. Dynamometric keyaccording to claim 1, characterized in that the turns of the spring (1),when the latter is in the free state, have a flattened shape along theradial axis (Or) thereof.
 3. Dynamometric key according to either one ofclaims 1 or 2, characterized in that the spring (1) is compressed alongits radial axis (Or).
 4. Dynamometric key according to one of thepreceding claims, characterized in that the securing element (5) isconstituted by a cylindrical element comprising a cylindrical outergroove (7) in which is fitted the spring (1), the base diameter (d) ofthe groove being slightly greater than the inner diameter (d′) of thespring (1) when it is in the free state.
 5. Dynamometric key accordingto claim 4, characterized in that the grip element (3) has a cylindricalrecess (18) hollowed out therein, whose diameter (D) defines, with thediameter (d) of the securing element (5), a spacing (D−d)/2 less thanthe height (e) of the turns when the spring (1) is in the free state, soas to compress them.
 6. Dynamometric key according to one of thepreceding claims, characterized in that the grip element (3) comprisestwo parts, namely a substantially cylindrical upper part (3 a) and alower part (3 b) substantially in the form of a bell.
 7. Dynamometrickey according to claim 6, characterized in that the internal volume ofthe bell-shaped part (3 b) is such that it has the capacity to envelopthe tool (22), whatever the positioning thereof inside the bell. 8.Dynamometric key according to one of the preceding claims, characterizedin that the hold, in the axial direction (yy′), of the securing element(5) with respect to the grip element (3), is ensured by the spring (1).